Hydrocarbon oils, particularly crude oils when produced comprise varying amounts of water and inorganic salts like halogens, sulfates and carbonates of Group I and Group II elements of The Periodic Table of Elements. (The Periodic Table of Elements referred to herein is the long form of the periodic table; Advanced Inorganic Chemistry by F. A. Cotton and G. Wilkinson Interscience Publishers, 1962.) Removal of water from produced crude oils is termed dewatering and salt removal is termed desalting. Often, the process of dewatering also desalts the crude oil since water-soluble salts are removed with the water.
Dewatering the produced crude oil is essential at crude oil production facilities as it impacts the value of crude oil and its economic transportation. The presence of salts, especially chlorides of Group I and Group II elements of The Periodic Table of Elements, corrode oil processing equipment. In order to mitigate the effects of corrosion, it is advantageous to reduce the salt concentration to the range of 1 to 5 ppm or less and water content to about 0.25 to 1 wt % by weight of the crude oil prior to transportation and processing of the oil.
Among the crude oil dewatering and/or desalting methods in use today, electrostatic separation methods are frequently used with crude oils containing about 0.5 to 12% water. In refinery desalting processes wash water is added to crude oil resulting in an emulsion having a water content is in the range of 4 to 8 vol %. Frequently a chemical emulsion breaker is also added to the emulsion. Then the emulsion is subjected to an electrostatic field so that, water droplets in the mixture of crude oil, wash water, and chemical emulsion breaker coalesce in the electrostatic field between electrodes. The coalesced water droplets settle below the oleaginous crude oil phase and are removed. Treated crude oil typically containing about 1 to 5 ppm inorganic salts is removed from the upper part of the separator.
One problem encountered with the electrostatic dewatering and desalting is that some crude oils form an undesirable “rag” layer comprising a stable oil-water emulsion and solids at the water-oil phase boundary in the desalter vessel. The rag layer often remains in the vessel but it may be removed therefrom for storage or further processing. Rag layers at the water-oil phase boundary result in oil loss and reduced processing capacity. Heavy crude oils containing high concentrations of asphaltenes, resins, waxes, and napthenic acids exhibit a high propensity to form rag layers.
Another problem associated with electrostatic desalting is that the brine droplets in crude oils are stabilized by oil components such as, asphaltenes, resins and naphthenic acids and the efficiency of desalting and dewatering is reduced. To overcome this problem U.S. Pat. No. 6,228,239 discloses use of certain chemical demulsifier formulations and optionally subjecting the crude oil and brine mixture or emulsion to opposed flow mixing prior to subjecting the mixture or emulsion to electrostatic desalting and or dewatering conditions. There is a continuing need for improved crude oil dewatering and/or desalting methods that improve the efficiency of dewatering and/or desalting especially with heavy crude oils containing asphaltenes and naphthenic acids. The present invention addresses this need.